Transmission techniques that support backbone networks of optical communication include an OTN (Optical Transport Network), which is standardized by the ITU-T (International Telecommunication Union Telecommunication Standardization Sector).
The OTN is a technique for accommodating client signals of SONET/SDH (Synchronous Optical Network/Synchronous Digital Hierarchy), Ethernet (registered trademark), or the like, to thereby generate a frame having a hierarchical structure, and performing WDM (Wavelength Division Multiplexing) transmission.
On the other hand, in recent years, there are increasing discussions of OTN evolution concerning how to accommodate various kinds of client signals while maintaining interoperability with the existing hierarchical structure to thereby realize efficient signal transmission.
One example of OTN evolution is a client signal-accommodating method using a LO (Lower Order)-ODU (Optical Channel Data Unit) and a HO (Higher Order)-ODU. This method improves flexibility of client signal accommodation by multiplexing and accommodating not only data signals of SONET/SDH, Ethernet, etc., but also e.g. signals having a frame structure, as client signals.
A frame for accommodating a client signal is called the LO-ODU, and a frame where the LO-ODU is stored is called as the HO-ODU. The LO-ODU is mapped in a payload area of the HO-ODU for transmission, and frame synchronization of the LO-ODU is detected for monitoring at a receiving end.
On the other hand, as a mapping method employed in accommodating a client signal in an ODU frame, a GMP (Generic Mapping Procedure) is standardized (ITU-T G.709/Y.1331). With the GMP, it is possible to accommodate a client signal with any bit rate less than the payload capacity of a frame into which the client signal is accommodated, and hence it is possible to support various client signals.
As a conventional technique, there has been proposed a mapping technique for the LO-ODU.
Japanese National Publication of International Patent Application No. 2010-541509
The GMP makes it possible to map (or demap) a client signal at a desired bit rate not higher than a payload capacity of a frame into which the client signal is accommodated, in a stepwise manner, but has no mechanism for detecting and reducing fluctuation in frequency (frequency deviation) of a transport signal during the mapping operation.
Therefore, in the OTN transmission apparatus, if the frequency of transport signals has a fluctuation exceeding the specification, the frequency fluctuation is propagated through transmission apparatuses by execution of mapping.
As described above, the conventional GMP method has a problem that even when a frequency fluctuation occurs during execution of mapping, the fluctuation is not compensated for at an intermediate pathway, and hence the frequency fluctuation is propagated to an apparatus at the other end, which degrades the transmission quality.